Code division, multiple access (CDMA) cellular communications provides 10-20 times the call handling capacity of analog cellular communications systems. See, e.g., "An Overview Of The Application Of Code Division Multiple Access (CDMA) To Digital Cellular Systems And Personal Cellular Networks," May 21, 1992, available from Qualcomm, Inc.--10555 Sorrento Valley Road--San Diego, Calif. A CDMA system transmits/receives voice or data at the relatively slow rate of approximately 8 Kbps to/from a mobile unit over a spread spectrum signal. (Other rates may be supported e.g. 16 Kbps, but the rate is still below the connection-based telephone industry standard rate of 64 Kbps). A transceiver at the cell site sends/receives the spread spectrum signal and translates it from/into CDMA data packets. A speech handler translates the data packets into and out of digital 64 Kbps pulse code modulated (PCM) stream, as used in standard connection-based digital switching. (CDMA cellular communications is more fully described in Qualcomm, Inc., "The Wideband Spread Spectrum Digital Cellular System Dual Mode Mobile Station-Based Station Compatibility Standard," and Qualcomm, Inc., "CDMA Digital Common Error Interface Standard," revision 1.0, October, 1993). In most CDMA system designs, the speech handler is at the cell site. A 64 Kbps circuit-switched connection is then made for transmitting and receiving CDMA data packets between the cell site through the serving switch and the public switch telephone network (PSTN).
A problem associated with this system is that, as the mobile moves from cell to cell, the circuit connection between the cell site and the switch must be torn down and re-established to the new cell site. Tearing down and re-establishing a circuit connection as a mobile moves from cell to cell and from switch to switch is known in the art as a "hard handoff." Hard handoffs are perceivable by the user as noticeable clicks and/or pauses in service. It is desirable in the art to provide "soft handoffs," or handoffs from cell to cell and from switch to switch that are not perceptible to the user.
One solution to the problem of hard handoff in CDMA systems is found in U.S. Pat. No. 5,184,347, which issued to AT&T in the name of Farwell, et al. (herein "Farwell"). Farwell provides a system that delivers CDMA packets from a cell site to a speech handler. Each cell site is connected to a cell-interconnect module which consists of a DS1 interface unit, a local area network bus, and a packet processing network. The content (voice or data) of the spread spectrum radio signal from the mobile unit is first packetized at the cell site. Packets are then addressed for the packet processing network and transmitted from the cell site over a DS1 link to the DS1 interface unit. This DS1 interface unit delivers the packets to the local area network bus, which is connected to a packet processing element. The packet processing element includes a table for translating a packet processing element address into a speech handler address. The packet is then sent to a optic fiber interface and over the optic fiber to an expansion interface. The expansion interface places the packet on another local area network. The packet is then picked up by a speech processing unit from the local area network.
Soft handoffs are initiated in the system of the Farwell patent by the mobile determining that it is in range of a second cell site. A handoff request message is sent from the mobile to the mobile switching complex, designating the second cell site as the new cell site. A new path is then set up through the switch complex to the speech handler by first selecting a route from the new cell site to the existing speech handler, and then informing each element along the route of the new connection. Each element then populates its lookup table with the appropriate call data to route the call to the next element, and eventually to the speech handler. The speech handler then receives two packets containing nearly identical data, and selects the one with a stronger signal strength (included as part of the data in the packet). Thus, the same speech handler is used for both cells, and a hard handoff is avoided. This system is complex in hardware and requires some time to establish and tear down each path through the system, because of the time necessary to populate all of the look-up tables.
A second solution to the problem of hard handoffs is found in U.S. patent application Ser. No. 08/040,819, filed Mar. 31, 1993, in the name of Hemmady, et al. (herein "Hemmady"). Hemmady provides a system for relaying packets between a cell site transceiver and a destination PSTN interface, via a self-routing, packet-switched mechanism. Packets from the cell site include a unique routing address of the destination PSTN interface (speech handler) which is located in a switching office. The packets are constructed by the receiver at the cell site including the content of the spread spectrum radio signal and sent in a packet pipe to a packet switch in a host switching system. A packet handler at the packet switch receives the packets, demultiplexes the packet pipe and forwards the packets on a packet bus. The destination PSTN interface assigned to that call recognizes its own address in the packets on the packet bus and processes the packets. As the mobile moves from cell to cell, and from a cell connected to one switch to a new cell connected to another switch, the new cell site is informed of the address of the destination PSTN interface originally assigned to the call, and the new cell uses its own packet pipes to its mobile telephone switch office (MTSO) and then to the destination PSTN interface, using the same address. The destination PSTN interface, during the period when a mobile is moving from one cell to another, receives packets from both cell sites and selects the best quality packet. This system does not require the circuit switched connections or updating tables, etc., of the Farwell disclosure. This system, however, requires expensive hardware for handling packets at each switch through which packets are sent.
Neither of these two systems address the problems associated with calls from a first CDMA mobile system to a second CDMA mobile system or provides soft handoff between multiple mobile systems. As stated above, CDMA digital packets are broadcast at a rate of 8 Kbps. When these packets arrive at a switch, they are translated by the speech handler into a 64 Kbps rate so that they may be handled by the switch. For a mobile-to-mobile call, the 64 Kbps packets must again be turned into 8 Kbps packets in order to be broadcast on the CDMA system. Every translation from one format to another degrades the quality of the encoded voice or other signals in the packet.
Therefore, a problem in the art is that there is no system which can provide soft handoff for a mobile to mobile call without degrading the quality of the call.